Concrete masonry units are typically produced using a production machine and a mold assembly. Generally, the mold assembly includes a mold having mold cavities and a tamperhead. The production machinery may drive the tamperhead into the mold to strip formed and compacted concrete products from the mold cavities.
The tamperhead is usually composed of several sub-components which may include an upper head structure, a plunger and a stripper shoe. Multiple sets of stripper shoes and plungers are connected to a single head structure and used to strip multiple masonry units from one or more molds or a set of concrete mold cavities. The plungers are commonly fabricated in structural shapes, depending on the shape and type of concrete units being formed. Plungers typically include a rigid material such as steel and are welded on one end to the head structure and on the other end to the stripper shoe. The plungers provide the structural load path to compress the concrete and strip the formed concrete product from the mold.
Upon filling the mold with concrete, the tamperhead is lowered until the stripper shoes enter the mold cavities and contact the concrete. To accomplish this, the stripper shoes and the mold cavities must be particularly aligned. This alignment process may inflict significant wear and stress on both the mold and tamperhead, resulting in increased production time and cost.
By design, a stripper shoe mounted on a tamperhead needs to fit a respective mold cavity with a minimal clearance. Depending on the type and size of product being manufactured, the amount of necessary clearance may vary. However, if the clearance is too small, the shoe will abrade against the cavity wall, thereby inducing stress in the mold and production machinery as well as premature wear on the machinery. On the other hand, if the clearance is too large, concrete will extrude between the shoe and the cavity walls, forming “burrs” on top of the product which, at best, detract from its aesthetic appeal and, at worst, create installation problems in the field.
Additionally, the demands and economics of the concrete product production industry result in the need to run the production machinery at high speeds to produce a large amount of product. As a consequence, a stripper shoe may impact the leading edge of a mold cavity repeatedly at high impact forces. The impact of the stripper shoe on the leading edge varies from one machine to another because different production machines use different systems to drive a compression beam. For example, some production machines use a hydraulic system, while others use a mechanical system. Additionally, the machines may operate at varying speeds and vibration frequencies as the specific type of concrete products being produced may require. Nevertheless, these impacts cause significant forces and vibrations in the mold cavities and in the tamperhead. These impact forces and vibrations are considered a significant factor in the failure of tamperheads and, more particularly, in the failure of plungers. Furthermore, severe impacts between the stripper shoes and the mold may cause significant damage to the mold and, in some cases, may result in catastrophic failure of the mold by crushing the thin walls separating the individual mold cavities.
While the tamperhead and the mold cavities are normally aligned when the production machinery is assembled, this alignment still does not prevent stresses on the tamperhead once the machinery begins to be used in the production of concrete products. The production process includes vibrating or shaking of the mold assembly with a vibration system every eight to fifteen seconds as the concrete is compacted. The vibrations serve to spread the concrete material evenly within the mold assembly cavities to produce a more homogeneous concrete product and assist in compacting the concrete product. However, these vibrations also transfer forces and stresses to the tamperhead, thereby causing small variations in the position of the production machinery.
Unfortunately, the repeated forces transmitted by the alignment impact forces and vibrations makes the plunger and joints in the tamperhead susceptible to material fatigue failure and cause wear and stress on the mold. As a result of the combined stresses and wear, expensive plungers typically last for only a short period of time and must be replaced at great expense and loss of valuable production time. Likewise, damaged mold cavities must be replaced or repaired, requiring significant and costly machining.
Furthermore, as the vibrator system shakes the mold assembly, the rest of the machine also experiences vibrations as forces are transmitted through the plungers. These vibrations cause the system to resonate if the frequency of the vibrator system approaches or falls on a natural frequency of the machinery. This resonance may fatigue the tamperhead, and particularly the plungers. As the components degrade, surface quality and product density of the finished product is also altered. Thus, the vibrations and alignment impacts reduce machine operating life and also reduce product quality and increase the frequency of replacement of parts.
A traditional approach for avoiding frequent failure and replacement of the plungers is to shorten the plunger length and increase the plunger strength and/or stiffness. However, this approach has not been successful at extending the useful life of the plungers. Time has shown that short, stiff plungers frequently fail, with the joint between the plunger and the head structure being especially vulnerable. Further, stiffer plungers increase wear on the stripper shoes and mold assemblies during alignment impacts and actually exacerbate the need to replace or repair expensive components.
Another traditional approach for avoiding frequent failure and replacement of the plungers is to increase the weight of the plungers. However, these plungers also increase production costs. As the weight of plungers increases, the expense of fabricating the plungers also increases due to the use of more expensive materials. Additionally, a plunger with greater weight functions to increase the power and expense required to run the production machinery. The increased weight also intensifies the deterioration of moving parts under heavy load and increased impact forces between stripper shoes and mold assemblies.
Therefore, there exists a need for a tamperhead which is less susceptible to failure due to vibration and fatigue stresses and which reduces impact loads between the mold cavities and stripper shoes during alignment.